Fuel injection nozzle

ABSTRACT

In a fuel injection nozzle having a nozzle body, a nozzle needle that is displaceable in the nozzle body, a piezoelectric actuator that is connected to the nozzle needle, and a compensation piston on which the piezoelectric actuator is braced and which protrudes into a compensation chamber that is filled with a fluid, where the compensation chamber communicates with a supply volume through an inlet of small cross section, a simpler design is to be attained. To that end, it is provided that the supply volume is subjected to variable pressure making it possible to dispense with a separate restoring spring for the nozzle needle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 00/03694 filed onOct. 20, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a fuel injection nozzle having a nozzle body, anozzle needle that is displaceable in the nozzle body, a piezoelectricactuator that is connected to the nozzle needle, and a compensationpiston on which the piezoelectric actuator is braced and which protrudesinto a compensation chamber that is filled with a fluid, wherein thecompensation chamber communicates with a supply volume through an inletof small cross section.

2. Description of the Prior Art

One such injection nozzle is known from German Patent Disclosure DE 3533 085 A1. The compensation piston makes it possible to compensate for achange in length of the piezoelectric actuator that ensues at low speed.A change in length of this kind is brought about in particular by achange in temperature. If such a change occurs, the fluid is positivelydisplaced by the compensation piston either out of or into thecompensation chamber. Conversely, if a rapid change in length of thepiezoelectric actuator occurs, as is brought about for opening of thenozzle needle, then because of the small cross section of the inlet,such high resistance to any displacement of the fluid present in thecompensation chamber is presented that the compensation piston functionsas a rigid abutment. The nozzle needle can then be actuated without thecompensation piston having any effect on the resultant opening stroke.

In the known injection nozzle, which is an outward-opening nozzle, aseparate restoring spring is provided for the nozzle needle. Opening thenozzle needle requires that the piezoelectric actuator overcome theforce exerted by the restoring spring, and this means strong actuationforces.

The object of the invention is to create a fuel injection nozzle thatmakes do without the restoring spring that has to be overcome by thepiezoelectric actuator.

SUMMARY OF THE INVENTION

A fuel injection nozzle of the type defined at the outset has theadvantage that the pressure prevailing in the supply volume, which alsoacts on the compensation piston, is comparable to a restoring spring forthe nozzle needle, so that a separate, strongly prestressed restoringspring can be dispensed with.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described below with reference to the single drawingFIGURE which is a schematic section of a fuel injection nozzle embodyingthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The injection nozzle has a nozzle body 10, in which an inward-openingnozzle needle 12 is displaceably disposed. The nozzle needle 12 can openinjection ports 14, which are embodied in the nozzle body 10, in orderto enable the injection of fuel, which is furnished via a delivery bore16 and an annular conduit 18. The delivery bore 16 is supplied from ahigh-pressure collection line known as a “common rail”.

On its end remote from the injection ports 14, the nozzle needle 12 isconnected to a piezoelectric actuator 20, which comprises a stack ofpiezoelectric elements 22 and a holder 24. The piezoelectric elementsare prestressed in the holder 24 by an annular spring 26. Thepiezoelectric actuator 20 is also provided with supply terminals, bymeans of which a voltage can be applied to the piezoelectric elements22.

On the end of the piezoelectric actuator 20 remote from the injectionports 14, there is a ball 28, on which a compensation piston 30 rests.The ball 28 assures that any tolerances and deviations in positionbetween the compensation piston 30 and the piezoelectric actuator 20 inthe radial direction will not be transmitted from one part to the other.The piston 30 protrudes into a compensation chamber 32, whichcommunicates with a supply volume 34 via an annular gap of thickness dbetween the wall of the compensation chamber 32 and the compensationpiston 30. The supply volume is at the system pressure P_(SYS), which istypically equal to the pressure of the furnished fuel.

Between the nozzle body 10 and a collar of the compensation piston 30,there is a compression spring 36, which urges the compensation piston 30toward the piezoelectric actuator 20. The compression spring 36 thusacts upon the compensation piston 30 in the direction of increasing thevolume of the compensation chamber 32.

The fuel injection nozzle described functions as follows: When theinjection system associated with the injection nozzle is turned off, thecompression spring 36 assures that the nozzle needle 12, via thepiezoelectric actuator 20, is pressed into contact with the nozzle body10, so that the injection ports 14 are closed, and no fuel from theinjection nozzle can enter the combustion chamber of an internalcombustion engine to be supplied. Conversely, if the injection system isturned on, and fuel which is under system pressure is furnished via thedelivery bore 16, an opening pressure is generated at the annularshoulder of the nozzle needle 12, which shoulder is disposed in theannular chamber 18. This opening force urges the nozzle needle 12upward, in terms of the drawing. The piezoelectric actuator 20 and thecompensation piston 30 act counter to the opening force. In theequalization state, a voltage is applied to the piezoelectric elements22, so that the individual piezoelectric elements lengthen axially. Inthis state, the rear end of the piezoelectric actuator 20 is braced, viathe compensation piston 30, against the fluid that is located at systempressure in the compensation chamber 32. When the nozzle needle 12 is tobe opened, the voltage applied to the piezoelectric elements 22 isinterrupted. The piezoelectric elements thereupon contract axially, sothat the nozzle needle 12 can execute an opening stroke of up to 0.25mm. During this motion, the compensation piston 30 remains in itsposition, since the fluid present in the compensation chamber 32 doesnot allow any motion of the compensation piston 30; the annular gaparound the compensation piston 30, through which the fluid could flow infrom the supply volume 34 for replenishing purposes, is so small thatthe system, for the brief periods of time while the nozzle needle isopen, can be assumed to be stationary. When the nozzle needle is to beclosed again, the voltage is again applied to the piezoelectric elements22, causing them to expand axially and displace the nozzle needle 12into its closed position.

If conversely during the operation of the injection nozzle, a change inlength of the components of the injection nozzle and in particular ofthe piezoelectric actuator occurs during a temperature change, thiscauses a corresponding displacement of the compensation piston 30 in thecompensation chamber 32; the fluid is then either positively displacedout of the compensation chamber 32 or aspirated into it through theannular gap having the thickness d.

The foregoing relates to preferred exemplary embodiment of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A fuel injection nozzle comprising a nozzle body(10), a nozzle needle (12) that is displaceable in the nozzle body, apiezoelectric actuator (20) that is connected to the nozzle needle, anda compensation piston (30) on which the piezoelectric actuator is bracedand which protrudes into a compensation chamber (32) that is filled witha fluid, and means connecting said compensation chamber for fluidcommunication with a supply volume (34) through an inlet of small crosssection and enabling a positive displacement of fluid out of or anaspiration of fluid into said compensation chamber (32) through saidinlet to compensate for a change in length of the piezoelectric actuator(20) during a temperature change, said supply volume (34) beingsubjected to variable pressure.
 2. The injection nozzle of claim 1,wherein said supply volume (34) is at the pressure of the fuel to beinjected.
 3. The injection nozzle of claim 2, wherein said supply volume(34) is filled with fuel.
 4. The injection nozzle of claim 3, furthercomprising a spring (36) which engages said piezoelectric actuator (20)and urges said nozzle needle (12) into its closed position.
 5. Theinjection nozzle of claim 4, further comprising a ball (28) disposedbetween the compensation piston and the piezoelectric actuator to enablean equalization of tolerances between said compensation piston and saidpiezoelectric actuator.
 6. The injection nozzle of claim 3, furthercomprising a ball (28) disposed between the compensation piston and thepiezoelectric actuator to enable an equalization of tolerances betweensaid compensation piston and said piezoelectric actuator.
 7. Theinjection nozzle of claim 2, further comprising a spring (36) whichengages said piezoelectric actuator (20) and urges said nozzle needle(12) into its closed position.
 8. The injection nozzle of claim 7further comprising a ball (28) disposed between the compensation pistonand the piezoelectric actuator to enable an equalization of tolerancesbetween said compensation piston and said piezoelectric actuator.
 9. Theinjection nozzle of claim 2, further comprising a ball (28) disposedbetween the compensation piston and the piezoelectric actuator to enablean equalization of tolerances between said compensation piston and saidpiezoelectric actuator.
 10. The injection nozzle of claim 1, whereinsaid supply volume (34) is filled with fuel.
 11. The injection nozzle ofclaim 10, further comprising a spring (36) which engages saidpiezoelectric actuator (20) and urges said nozzle needle (12) into itsclosed position.
 12. The injection nozzle of claim 11, furthercomprising a ball (28) disposed between the compensation piston and thepiezoelectric actuator to enable an equalization of tolerances betweensaid compensation piston and said piezoelectric actuator.
 13. Theinjection nozzle of claim 10, further comprising a ball (28) disposedbetween the compensation piston and the piezoelectric actuator to enablean equalization of tolerances between said compensation piston and saidpiezoelectric actuator.
 14. The injection nozzle of claim 1, furthercomprising a spring (36) which engages said piezoelectric actuator (20)and urges said nozzle needle (12) into its closed position.
 15. Theinjection nozzle of claim 14, further comprising a ball (28) disposedbetween the compensation piston and the piezoelectric actuator to enablean equalization of tolerances between said compensation piston and saidpiezoelectric 3 actuator.
 16. The injection nozzle of claim 1, furthercomprising a ball (28) disposed between the compensation piston and thepiezoelectric actuator to enable an equalization of tolerances betweensaid compensation piston and said piezoelectric actuator.